Islamic State (IS) militants have committed “mass atrocities” in Syria and have recruited children as fighters, the United Nations says.
In a report, investigators say public executions are a “common spectacle” in areas run by IS, one of the groups fighting against Syria’s government.
The report also accuses the Syrian authorities of using chemical agents in eight separate incidents this year.
The conflict between government forces and several rebel groups began in 2011.
Some 200,000 people have died since then.
The UN report details abuses by the Syrian government and several of the armed groups fighting it. The report says the Syrian air force has used barrel bombs on civilian neighbourhoods.
“In some instances, there is clear evidence that civilian gatherings were deliberately targeted” by government forces, the investigators said.
“In government prisons, detainees were subjected to horrific torture and sexual assault.”
The findings are the result of interviews and evidence collected between January and July this year as part of an inquiry into human rights violations in Syria.
Among the other allegations of war crimes committed by the Syrian government of President Bashar al-Assad was the use of suspected chlorine gas, a chemical agent, in eight separate incidents in April and May of this year.
The period covered in the report coincides with the growth of IS in Syria. The group seeks to create an independent Islamic State in an area that stretches across Syria and Iraq.
It has attracted jihadists from across the region, as well as fighters from Western countries including the UK and the US.
Analysis: Barbara Plett, BBC News, Washington
The brutal murder of American journalist James Foley last week has focused minds here on a broader response to the rapid advance of IS.
The US is considering expanding its campaign of limited air strikes in Iraq, the most immediate possibility being a humanitarian relief operation for Shia Turkmen besieged by IS militants in the northern part of the country.
US spy planes have also begun surveillance flights over Syria to identify potential IS targets, after America’s top general said the militant group could not be defeated without attacking its Syrian bases.
US officials say that President Obama has not yet decided whether to authorise direct military intervention in Syria, which would be a significant escalation. But they all stress that any campaign to root out IS in Iraq and Syria would be long and difficult and would need to include increased support for local armed forces as well as a political solution.
It would also need to be backed by a coalition of European and regional countries, which the Obama administration has begun trying to mobilise.
Training child soldiers
In their report, UN investigators said IS was waging a campaign of fear in northern Syria, including amputations, public executions and whippings.
“Bodies of those killed are placed on display for several days, terrorising the local population,” the document says.
“Women have been lashed for not abiding by IS’s dress code. In Raqqa, children as young as 10 are being recruited and trained at IS camps.”
On Wednesday IS supporters tweeted pictures allegedly showing militants executing Syrian army soldiers after capturing the government Tabqa airbase near Raqqa in eastern Syria. The pictures have not been verified.
Paulo Pinheiro, the chairman of the UN panel, said the international community has failed “in its most elemental duties – to protect civilians, halt and prevent atrocities and create a path toward accountability”.
One of the investigators, Carla del Ponte – a former chief prosecutor of two UN war crimes tribunals – has urged world powers to refer Syria to the International Criminal Court.
In a separate development, Syrian rebel groups including the al-Qaeda affiliated Nusra Front have taken control of a crossing between Syria and the Israeli-occupied Golan Heights, according to a UK-based monitoring group.
“The Nusra Front and other rebel groups took the Quneitra crossing, and heavy fighting with the Syrian army is continuing in the surrounding area,” said Rami Abdel Rahman, the director of the Syrian Observatory for Human Rights.
The Israel Defence Forces said one of its officers “was moderately injured as a result of errant fire from Syria”.
“In response, we struck 2 Syrian military positions in the Golan Heights,” an IDF spokesperson